The overall goal of this Interactive Research Project Grant (IRPG) is to develop new antiviral nucleosides, for use in combination against human immunodeficiency virus (HIV) and hepatitis B virus (HBV) infections. As a first step, the applicant wishes to focus on a new compound discovered in his laboratory that has dual potent and selective activity against HIV/HBV. b-D-2',3'-Didehydro-2',3'-dideoxy-5-fluorocytidine (D-D4FC) and related molecules will be studied extensively at the biochemical, virological, molecular therapeutical, and pharmacological level in cell-free and cellular systems. The activity of D-D4FC will be tested against a panel of well characterized clinical isolates of HIV in primary human lymphocytes. HIV-1 resistant to other nucleoside analogs will also be tested for cross-resistance to D-D4FC. Based on the cross-resistance pattern, D-D4FC will be evaluated in double and triple combinations with AZT, 3TC, (-)-FTC, d4T, ddI, ddC, a protease inhibitor such as indinavir, and a nonnucleoside reverse transcriptase inhibitor, such as nevirapine. Moreover, the relative rate for the emergence of resistance to D-D4FC will be determined and compared with 3TC [or (-)-FTC] and AZT. D-D4FC is currently being evaluated in a woodchuck hepatitis virus model by the NIAID. This application proposes to conduct combination studies of D-D4FC with 3TC, DAPD and (-)-FTC, compounds with dual anti-HIV and anti-HBV activity in HBV transfected cells. The potency and selectivity of the 5'-triphosphate of D-D4FC will be compared to the L-enantiomer and related nucleotides by studying the inhibition of the HIV-1 reverse transcriptase, the HBV DNA polymerase, and several human DNA polymerases. Radiolabeled D-D4FC and L-D4FC will be synthesized and their cellular pharmacology will be determined in order to gain insight into their different biological properties.